hardMultiple Choice
300-410 Practice Question: An engineer configures a distribute-list on an…
An engineer configures a distribute-list on an OSPF router to filter routes. However, the routes are still being advertised to neighbors. Which is the most likely explanation?
⚠ Common exam trap
The trap is assuming that OSPF distribute-lists filter routing updates like in EIGRP or RIP. Candidates must remember that OSPF is link-state and distribute-lists only affect local RIB installation, not LSA flooding.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The distribute-list filters routes in the routing table, not the LSAs sent in OSPF updates.
In OSPF, a distribute-list applied under the OSPF process filters routes from being installed in the local routing table (or from being redistributed), but it does not filter LSAs from being flooded to neighbors. OSPF is a link-state protocol; once an LSA is originated, it is flooded throughout the area regardless of local distribute-list filtering. Therefore, routes still appear in neighbors' LSAs and can be installed if the SPF calculation permits.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The distribute-list filters routes in the routing table, not the LSAs sent in OSPF updates.
Why this is correct
OSPF distribute-lists applied to routing updates filter routes entering the local routing table, not the link-state advertisements flooded to neighbours. Because LSAs are synchronised via the link-state database, neighbours still receive and install the routes.
- ✗
The distribute-list is applied to the wrong interface.
Why it's wrong here
OSPF distribute-lists filter routes redistributed into OSPF, not routes learned via OSPF and flooded to neighbours, so interface application cannot suppress them. It is tempting because distribute-lists on interfaces do filter incoming and outgoing updates in distance-vector protocols such as RIP or EIGRP, where that placement is correct.
- ✗
The distribute-list uses an incorrect prefix-list.
Why it's wrong here
An incorrect prefix-list would filter the wrong prefixes rather than leave all routes advertised, so it cannot explain the symptom. It is tempting because prefix-list entries are the match criteria a distribute-list references, and a typo there would be the fault if some routes were filtered but others unexpectedly passed.
- ✗
The OSPF process needs to be restarted for the distribute-list to take effect.
Why it's wrong here
OSPF applies distribute-list changes immediately; no process restart is needed, so this cannot be the cause. It is tempting because some IOS features require a clear or reload to take effect, and restarting OSPF is a familiar troubleshooting step when configuration changes appear not to apply.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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